1. Field of the Invention
The present invention relates to extractants for palladium and a method of separating and recovering palladium. In particular, the present invention relates to extractants which can rapidly extract palladium and can back-extract palladium using an ammonia solution, and to a method of separating and recovering palladium using the extractants.
2. Description of the Related Art
In recent years, the demand for autocatalysts has been enhanced worldwide with the tightening of regulations regarding the emissions from automobiles. Since the autocatalysts contain a significant amount of platinum group metals, like palladium, platinum, and rhodium, the improvement of the separation and purification of the metals from their wastes, along with metal refining from ores, has become important, in order to ensure stable supply of those platinum group metals.
For separation and purification of the platinum group metals in practical processes, various methods such as a sedimentation separation method (JP 10-102156 A), an ion exchange method (JP 03-22402 A and JP 07-310129 A), an electrolytic precipitation method (JP 08-158088 A), and a solvent extraction method have been conventionally proposed and carried-out. Of those methods, the solvent extraction method has been widely adopted in view of economy and operability.
For the separation of palladium by the solvent extraction method, it is required that a selective separation of palladium over platinum and rhodium, and over base metals when possible. Di-n-hexylsulfide (DHS) is one of the most commonly used industrial extractants for palladium (JP 08-209259 A, JP 09-279264 A, JP 2001-107156 A, JP-2004-332041 A, and JP 63-14824 A). DHS is capable of selectively extracting palladium from an acidic aqueous solution containing palladium, platinum, and rhodium, and then palladium can be recovered from the DHS solution containing palladium using an ammonia solution. However, the extraction rate of palladium is small when DHS is used alone; therefore, the addition of another extractant to DHS is required to improve the extraction rate (JP 63-14824 A).
Sulfur-containing diamides have been proposed as palladium extractants which can rapidly extract palladium with a high selectivity (WO 2005/083131), but it is difficult to back-extract palladium using an ammonia solution, and the back-extraction requires the use of a hydrochloric acid solution containing thiourea. The use of the hydrochloric acid solution containing thiourea is not necessarily suitable for practical application; therefore, the back-extraction using an ammonia solution is needed.